WO2010132927A1

WO2010132927A1 - Forward looking borehole radar to determine proximity of adjacent interface of different seams or layers

Info

Publication number: WO2010132927A1
Application number: PCT/AU2010/000583
Authority: WO
Inventors: Iain Mason
Original assignee: Geomole Pty Ltd
Priority date: 2009-05-20
Filing date: 2010-05-18
Publication date: 2010-11-25

Abstract

Forward looking radar is employed to sense drill bit approach to an interface between different layers during drilling. A drill bit of a drill string is electromagnetically excited. Electromagnetic reflections from the drill bit are received during drilling. The received electromagnetic signal is processed to determine a proximity of the drill bit to an interface between different layers during drilling. Geostopping may thereby be effected to provide for the drill bit to stop a desired distance before piercing the interface. Near-bit sensing of evanescent seismic fields may also, or alternatively, be used for geostopping and/or other purposes.

Description

FORWARD LOOKING BOREHOLE RADAR TO DETERMINE PROXIMITY OF ADJACENT INTERFACE OF DIFFERENT SEAMS OR LAYERS

Technical Field

The present invention relates to a method and system to electromagnetically sense during drilling an interface between different rock layers ahead of the drill bit, prior to piercing of the interface by the drill bit. In particular the invention relates to a method and system to electromagnetically sense the interface of a coal seam with an overlying layer, when drilling overburden removal blast holes.

Background Art

Blasting damage to a coal seam during open-cut overburden removal can cause coal losses of greater than 10%, and can cause greenhouse gas losses. Industry investigations have shown that blasting damage could be cut and methane escape arrested if all blast-holes could be stopped short of the top of a coal seam. However, seam depth is sometimes hard to predict. This makes it difficult to use dead reckoning to drill to a predetermined position above the top of the seam.

Current methods for predicting the local depth of a coal seam involve extensive pierce point drilling and geophysical logging of exploration holes prior to overburden removal. Exploration holes are rarely at spacings closer than 50m x 50m, and often at 100m x 100m spacings or more. However, blast holes are typically at a spacing of around 10m x 10m, requiring interpolation of the formation between the exploration hole pierce points. This exploration drilling is costly and time consuming. Further, significant inaccuracies regularly arise during interpolation when, for example, the targeted coal seam's top rolls; when faults suddenly shift the seam top's elevation, when (say) layers are washed out, or sandstone lenses are created during the deposition of the overburden. Similar variations in overlying marker layers tend to limit the reliability of referring to such overlying marker layers for position determination. Any discussion of documents, acts, materials, devices, articles or the like included in the present specification is for the purpose of providing a context for the present invention, and is not to be taken as an admission that any such matters form part of the prior art base or were before the priority date of each claim of this application common general knowledge in the field relevant to the present invention.

In this document the term "comprise", and derivatives including "comprises", "comprised" or "comprising", are to be understood to convey inclusion of one or more stated elements, integers or steps, but not the exclusion of any other element, integer or step.

Summary of the Invention

According to a first aspect the present invention provides a method for sensing an interface ahead of the drill bit between different layers during drilling, the method comprising: electromagnetically exciting a drill bit at the end of a drill string; detecting impedance changes near the drill bit during drilling; and processing detected impedance changes to determine a proximity of the drill bit to a targeted interface between different layers during drilling.

According to a second aspect the present invention provides a system to sense an interface between different rock layers ahead of a drill bit during drilling, the system comprising: a transducer to electromagnetically excite a drill bit at the end of a drill string; a sensor to detect impedance changes near the drill bit during drilling; and a processor to process detected impedance changes to determine a proximity of the drill bit to a targeted interface between different layers during drilling.

The present invention recognises that electromagnetic excitation of a drill bit generates energy fields beyond, or ahead of, the drill bit, These fields may propagate away from the drill bit, or they may be bound to it evanescently. The electromagnetic impedance of the drill bit is thus influenced by dielectric and resistivity changes at geological interfaces proximal to and ahead of the drill bit. Thus, detecting and analysing changes to the electromagnetic impedance, either monochromatically, at spot frequencies, or over a broad spectrum in say the HF-VHF band, or near-equivalently in the time domain as a drill bit progresses, can enable blast hole drillers to monitor and control the bit's drill approach to a targeted interface, even if its absolute elevation is uncertain.

Suitable selection of electromagnetic excitation can make the technique of the present invention sensitive to a desired range ahead of the bit. For example an excitation at 27 MHz in dry sandstone or siltstone may yield sensitivity in the range of tens to hundreds of centimetres ahead of the drill bit, for example to enable geostopping at such ranges before the drill bit pierces an interface.

The drill string may be excited by a borehole radar (BHR) that inductively couples guided EM waves to the drill steel, either from a proximal resistively loaded antenna that parallels the drill string's axis, or a toroid that causes a B field to flow around the circumference of the drill rod. The EM signal may be remotely coupled into the drill string for propagation to the drill bit. Given that blast hole drill strings at less than 100m are comparatively short the EM signal may even be inductively coupled onto the drill string by a proximal antenna or a toroid at the surface. Alternatively, a section of steel near the drill bit may be driven into resonance by driving a proximal antenna or a toroid located near the bit using a down-hole EM signal source. The frequency of such a source and its position relative to the bit may be chosen, for example to facilitate energy trapping in the space ahead of the bit by for example quarter wavelength resonance.

The method may further comprise providing a receiver for receiving the reflected electromagnetic signals. Similar options to those available in EM transmission are available in the selection and positioning of sensing elements

Preferably the received electromagnetic signals are analysed in real time.

Processing the received electromagnetic signals may include detecting whether the drill bit is approaching a coal seam interface. In the event that the drill bit is approaching a coal seam interface the method may further include sensing when the drill bit is at a predetermined distance from the interface, and generating an alert to alert a driller operating the drill string of the proximity of the drill bit to the coal seam interface. The alert signal may be derived from a comparison, either mathematically based or subjective, of patterns emerging from the present blast-hole with those that were recorded in earlier blast-holes.

The method may further comprise providing a graphical user interface at the surface to display images of the measurements-while-drilling on a user display. Computationally derived distances-to-target might also be signified to the blast hole driller by changes to the pitch or mark-space ratio of an audible "beep"

The electromagnetic excitation for example may be at a frequency selected to excite evanescent wave resistivity (EWR) fields ahead of the bit. For example the excitation may be at HF or VHF frequencies and for example may be around 27 MHz or greater. Such embodiments may be particularly advantageous in a relatively insulating formation, such as dry rock. Additionally or alternatively, the electromagnetic excitation may be at relatively low frequency such as 200 kHz or less, such as 50 Hz, to excite ohmic current fields ahead of the bit. Such embodiments may be particularly advantageous in a relatively conductive formation, such as water laden rock.

In embodiments where the drilling fluid is resistive, such as where air is the drilling fluid, one of the transmitter and receiver may be positioned top-hole. For example one of the transmitter and receiver may be provided at the surface, for example comprising a toroidal coil mounted on a drill rig and positioned around a top of the drill string and configured to inductively couple electrical energy to or from the drill string. Such embodiments recognise that signals - particularly low frequency (eg 50-2500Hz signals - coupled onto the drill string will suffer minimal or at least acceptable losses between the drill bit and the surface, as the primary electrical contact between the drill string and the surrounding formation often will be the near-to-bit guide fins of a drill string stabiliser and/or the drill bit itself. This is in contrast to drilling with conductive drilling fluids, such as water or oil, in which it is to be expected that excessive losses will occur laterally from the drill string into the formation and will prevent EM signals coupled onto the drill string from propagating between down-hole and top-hole. However, even in circumstances where the air or other drilling fluid does not adequately insulate the drill string from the formation and current shunts radially into the formation, the look-ahead effect of the present invention might still be obtained by obtaining a down-hole measure of the current which actually passes through the drill bit, for example using a toroid positioned at the drill bit.

Formation-induced changes in the evanescent field may be detected by electrical resistance-at-bit measurements, for example at or below 200 kHz. They may also be detected by comparing electromagnetic measurements and recordings at different frequencies and/or in different boreholes either subjectively or by other more formal means of pattern recognition. Embodiments of the invention may utilise seismic geostopping, whereby the output signals of accelerometers positioned near the bit are processed in order to detect an elastic response of the formation to the action of the bit during drilling, whereby changes in the detected elastic response herald approach of the bit to a layer interface, permitting geostopping prior to piercing of that interface. Such seismic geostopping may be employed alone or in conjunction with electromagnetically enabled geostopping. Accordingly, in a third aspect the present invention provides a method for sensing an interface ahead of the drill bit between different layers during drilling, the method comprising: obtaining seismic measurements near the drill bit during drilling; and processing the seismic measurements to detect seismic impedance changes arising from an interface ahead of the drill bit, to determine a proximity of the drill bit to a targeted interface between different layers during drilling.

Such embodiments of the invention recognise that conventional seismic-while- drilling (SWD) approaches utilise surface detectors to detect p-wave seismic signals from the action of the drill bit. Due to ground attenuation such previous SWD techniques must operate at low frequencies such as less than 100Hz. In contrast, near-bit seismic detectors such as accelerometers are likely to receive seismic signals at significantly higher frequencies allowing much improved position resolution, particularly in water laden formations in which elastic signals of many kHz or tens of kHz may effectively propagate over the ranges of interest to the present application.

In the first to third aspects of the invention the processing may involve auto- correlating signals received over time, so as to detect changes in the formation response as an interface is approached. Control signals and the like may be communicated down-hole, and/or logged data signals and the like, may be communicated up-hole, by any suitable communications-while-drilling technique.

According to a further aspect, the present invention provides a method to sense the interface of different rock layers during drilling, the method comprising: guiding a drill bit attached to a metallic drill string down a borehole; exciting electromagnetic transmissions which couple onto the metallic drill string; receiving reflected electromagnetic signals from the drill bit as it drills said borehole; and processing the received electromagnetic signals to determine the proximity of a tip of the drill bit to an interface of different rock layers during drilling.

According to another aspect the present invention provides a system to sense the interface of different rock layers during drilling, the system comprising: guide means to guide a drill bit attached to a metallic drill string down a borehole; a transmitter to excite electromagnetic transmissions; coupling means to couple the excited electromagnetic transmissions to the metallic drill string; a receiver to receive reflected electromagnetic signals from the drill bit as it drills said hole; an antenna; and a processor to process the received electromagnetic signals to determine the proximity of a tip of the drill bit to an interface of different rock layers during drilling.

Brief Description of the Drawings Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1a illustrates an experimental set up wherein a borehole radar is deployed in a borehole adjacent to conductive wire at a distance there from of 2m. The borehole radar is inductively coupled to the conductive wire;

Figure 1b illustrates the resultant borehole radar profile obtained from the experimental set up shown in Fig. 1a;

Figure 2a is a schematic illustration of electromagnetic logging while drilling in accordance with the invention;

Figure 2b is a finite element simulation which illustrates the waveform of the reflected electromagnetic wave obtained from the set up shown in Fig. 2a, and how it changes as a function of proximity to the varying rock layers;

Figure 3a illustrates a drill string stabiliser;

Figure 3b illustrates modifications to the drill string stabiliser of Figure 3a to implement one embodiment of the present invention; and

Figure 3c is an exploded view of a portion of Figure 3a.

Description of the Preferred Embodiments

The present invention recognises that in resolving depth or stratigraphic position of a drill bit at sub-wavelength scale during drilling, neither elastic (seismic) sensing nor electromagnetic sensing necessarily needs to be temporally or positionally remote from the drill bit. Currents, or in dry rock EM modes guided by drill strings, blow potential fields, or evanescent EM energy into bubbles ahead of drill bits. These elastic and electrical bubbles distort dynamically as the drill-bit closes on its objective. Moreover, drilling itself drives evanescent elastic energy ahead of a drill bit.

Preliminary computer simulations, backed by field trials with borehole radar transceivers in sandstone boreholes (shown in Fig. 1a) indicate the possibility to retrofit an existing VHF-UHF borehole radar system onto a blast-hole drill rig, and to automate the processing of rod-guided radar echoes in real time in such a way as to warn the driller automatically (with better than a +/- 0.5m precision) of the drill-bit's approach to a contrasting interface near the top of coal interfaces, say between sandstone and siltstone.

Fig. 1b illustrates the results of such field trials which were backed by finite- difference time-domain modeling. The reflected electromagnetic waves from the top and bottom of the wire are clearly recorded by the borehole radar shown in Fig. 1a. These results indicate that existing VHF-UHF borehole radar technology (such as that disclosed in PCT/AU2004/001382 and PCT/AU2005/001201 , the contents of which are herein incorporated in their entirety) can be adapted to the task of guiding blast hole drill bits to a precise (+/- 0.5m) stop at or near 30-10Om deep target horizons such as the sandstone-siltstone interface that lies above many coal seams.

Fig. 2a illustrates a schematic illustration of the proposed electromagnetic logging while drilling radar (EM LWD). When an electromagnetic wave travels down along the drill string, reflection of the drill bit can be thought of as causing EM evanescent energy to hang for an instant like a bubble off the drilling end of the drill string, before it collapses in order to generate the echo. As is shown in Fig. 2b, the energy and shape of the reflected 10MHz λ~10m electromagnetic wave, changes dramatically over the final 2.5m of drill progress towards an interface, slumping as the siltstone covering the top of coal is approached and energy is lost to interface waves that attenuate as they sweep radially outwards across the top-of-silt. Finally, upon touch-down of the drill-bit upon the silt, the reflected wave reverses phase as E fields at the bit are suddenly shorted out (see the inset box at top right in Fig. 2b).

The present embodiment of the invention recognises that rotary air blast (RAB) drill strings used in blast hole drilling are short, generally being less than 60m, and that the drilling fluid (air) is usually insulating. The drill steel's electrical resistance is very low. Thus, at least to a degree, the path from the drill rig on the surface to the drill bit is insulated from the formation drilled, by the air-slurry that is blown up the annulus between the borehole wall and the drill steel. The present embodiment thus recognises that it is possible and economical to launch all of the electrical power that is needed to drive a rotary air-blast look- ahead transmitter from a surface toroid (or group of toroids), such as an air or ferrite or ferromagnetically cored Rogowski coil, which surrounds the drill steel between the drill rig and the entrance to the developing borehole.

In such an arrangement, it is to be expected that some current will shunt directly radially outwards from the drill steel into the formation. However, this shunt current may be limited, particularly in rotary air blast hole drilling, because there drill strings are short, and the drill string is immersed in a cutting- filled air-based annulus. The top-hole fed electrical power supply can be significant, and it has been shown that some current will reach the drill bit and pass through the drill bit into the rock. The approach of the drill bit to a contrasting horizon can then be sensed by changes to this, the plume or look- ahead current, using a downhole receiver, that is fed by a toroid which is mounted on or near the drill bit as shown in Figure 3.

The present embodiment of the invention thus provides for a toroid and/or other suitable sensors/transducers to be positioned at or near the stabiliser and drill bit. Figure 3a illustrates a typical drill string stabiliser. Stabiliser 300 comprises a hollow core 302 for air flow, upper male threaded portion 304 to connect to a drill string and lower female threaded portion 306 to connect to a drill bit. Stabiliser fin 308 extends radially outwards from the stabiliser for the purpose of making contact with the walls of the borehole.

Figure 3b is a cross sectional exploded schematic of the stabiliser 300, focused on the lower rightmost portion of the female threaded portion 306. Stabiliser 300 is modified as shown in Figure 3b to accommodate a transceiver and control unit 310, recessed into the wall of the stabiliser core 302. Control unit 310 is of annular form positioned completely around a circumference of the core 302. Such positioning of the control unit 310 protects sensitive electronics, battery, and communications components from the rigours of the down-hole environment. Control unit 310 exploits a number of sensors and transducers 312 in annular positions on the outer wall of portion 306. Components 312 are spaced by annular spacers 314, with the bank of spacers 314 and elements 312 being built by sliding each annular element onto the stabiliser 300 from the drill bit end. The bank of spacers and elements are secured in place by annular shields 316, which are formed of hard wearing material such as steel and secured to the stabiliser 300 in a manner to withstand the forces applied to the leading edge of the stabiliser by rock chips and the like passing upwardly in the air blast. Once the bank of sensors and/or transducers 312 are in place, wiring from each is passed through channel 318 to connect the control unit 310. In one embodiment transducer 312a is a VHF transmitter coil, transducer 312b is a natural gamma ray detector and transducer 312c is a VHF receiver coil, with the control unit using a transmit/receive switch of the type set out in International Patent Publication No. WO 2006/015436. Transducer 312d is an iron-cored toroid which senses low frequency electron flow into the drill bit, Fin 308 is positioned sufficient distance up-hole to make room for components 312, 314.

Figure 3c is an exploded cross sectional schematic of a single annular transducer 312d from Figure 3b. As can be seen the active element 322 is protected from the stabiliser portion 306, the annular shield 316 and from the borehole environment by a cladding 314 The fact that cladding 314 is an insulator prevents currents short circuiting the toroid 322 Cladding 314 is thickened where exposed to the borehole as the toroid 322 must survive in an abrasive dense stream of rock fragments, travelling at around 100 kph. Cladding 314 may comprise an epoxy, malleable plastic compound and/or fibreglass seal. Notably, when the drill string and stabiliser 300 are withdrawn from a borehole, the components 310, 312 will be brought proximal to a top-hole coil. Thus, between drilling periods, a battery of control unit 310 may be inductively charged by inductively passing electrical power between the top-hole rig- mounted coil and the stabiliser-mounted coils .

The components 312 may comprise toroidal coils for transmitting and/or receiving, Wu-King wideband antennas, VHF transceivers, natural gamma radiation detector, a ferrite ceramic ring such as Barium Titanate for ultrasonic excitation around 20 kHz, an inductive electromagnetic detector, or other sensor or transducer.

It is further noted that VHF electromagnetic energy in rock is typically considered to have a resolution limit of around three meters. This is because VHF electromagnetic frequencies typically used in ground penetrating radar are those which propagate adequately through subsurface formations, and generally have a wavelength of several metres or more, with the imaging resolution being about the same as the wavelength. However, it is to be noted that the technique of some embodiments of the present invention utilize non- propagating evanescent electromagnetic energy. This provides for sub- wavelength look-ahead resolution at interfaces, as illustrated in Figure 2, and may thus provide imaging or stratigraphic positioning resolution of less than a metre and preferably less than about 0.2 m. Such positioning resolution is of much importance in drilling blast holes.

The present invention further recognises that two forms of current may be exploited in this context: ohmic or resistance-at-bit (RAB), & displacement or electromagnetic wave resistivity (EWR). Depending on the borehole, either or both may be relevant. In conducting (e.g. water laden) rock, low frequency plumes of ohmic current must be driven ahead of the bit by axial currents that are induced in the drill string. In insulating (e.g. dry) rock, HF/VHF borehole radars may be used to feed transient evanescent EM bubbles that form during the reflection of the broadband EM pulses that are guided by the drill string. Down-hole components for ohmic techniques may be mounted as shown in Figure 3.

The present invention thus involves coupling an electromagnetic signal onto a drill string and then measuring the signal response as the drill bit attached to the drill string grinds through rock mediums of varying electrical properties. Computer simulations, backed by field data with borehole radar transceivers in sandstone indicate that the signals do propagate along drill strings, and that the nature of the reflected signals are influenced by the medium immediately surrounding the tip of the drill. Thus it is proposed that such a method and system in accordance with the invention will enable a driller to distinguish the difference between say sandstone and siltstone whilst drilling a borehole.

Electromagnetic theory suggests that the resolution given by this technique will be a fraction of the transmitted wavelength, so that even with a wavelength of 3m it should still be possible to achieve <0.5 m resolution accuracy. This will enable the system to measure and locate the various stratigraphical rock layers to the accuracy demanded by the blast engineers.

Further embodiments of the invention may additionally or alternatively provide for elastic geostopping. This is in recognition that rock ahead of a drill bit strains measurably under the load imposed by the weight on the bit. Dynamic elastic and plastic strains are caused by vibration and the cutting action of the bit during drilling. Such embodiments thus provide for vibration sensors to be located at or near the drill bit, for example as shown in Figure 3. In contrast to previous seismic-while-drilling (SWD) approaches relying on propagating seismic components, such embodiments of the present invention also exploit non-propagating seismic variations around the drill bit. Such embodiments of the present invention thus further exploit the recognition that drilling bits can be surrounded by (at least) two evanescent volumes: one mechanical, one electrical, each of which can have static and/or dynamic components. The material underlying each of the benches that separate the seams in a typical coal deposit was built up by the sedimentation of sands and clays that were deposited during episodic inundations of varying energy. The coals, silts/shales, and sandstone layers have contrasting mechanical properties. The volume of mechanical energy, some evanescent, some static, some dynamic and in transit, that surrounds each drill bit changes its shape and state with depth. Some of this evanescent volume extends in front of the bit, which is why the thud of the downhole hammer, (in this case built into a tricone's rollers), changes before the bit bites into clay.

The static form of the mechanical or seismic field contains rock that strains measurably under the load imposed by the weight on the bit. The electromagnetic field can contain a static potential field introduced by charge applied to the drill steels which measurably permeates a near-bit volume of rock. Embodiments of the invention may exploit that, as the drill bit closes on an interface such as a hard sandstone band embedded in weak sandstone, there is a change in the "thud" of a drill bit. Moreover, the present invention exploits an electrical analogue to this near-interface effect.

Dynamic, (i.e. vibrational or non static) elastic and plastic strains in the mechanical evanescent volume are driven by vibration and the cutting action of the bit. Analogous dynamic, evanescent electrical fields arise near the bit if alternating currents are driven into the drill steels as is provided for by the present invention. In both mechanical and electromagnetic cases, the evanescent fields contribute to reflecting downgoing energy back up the drill bit. In both mechanical and electromagnetic cases, insofar as symmetry permits, down-going energy can be transmitted through the evanescent volume to propagate freely out into the space surrounding the bit.

Thus, vibration sensors in the drill bit such as accelerometers or geophones, installed in the manner set out in Figure 3, can be used in such embodiments to sense the oscillatory part of the mechanical field that surrounds the bit. Freely propagating, reflected and evanescent wave-fields all contribute to this field . Electrical sensors such as toroids and/or radar receivers installed as shown in Figure 3 similarly sense the oscillatory part of the electrical field that surrounds the bit. Circumferential and axial accelerometers built into or near the bit are able to sense some of the interfacial changes of elasticity, that make up mechanical contrast. Other transducers, namely low frequency toroids that surround the bit, are designed to pick up electrical changes in the plumes of low-frequency ohmic currents that can be forced into conducting rocks ahead of the bit. High frequency displacement currents generated by radar play a similar role in insulating rocks through which low frequency ohmic plumes do not flow.

Each type of sensor will respond in its own way to any changes in the near-to- bit fields, caused by mechanical or electrical reflections as the drill bit passes or approaches inclusions in the host rock, and/or may be linked in to a sedimentary sequence of overlying marker layers to the approach of the drill-bit to an interface.

The different forms of evidence of change in near-to-bit energy storage, that are offered by accelerometers, geophones, toroids, and/or radar receivers, can be combined to guide the drill bit's approach to a chosen target. Thus, to improve contrast the present embodiment provides for multisensor integration. Expert systems can be provided to integrate the evidence offered by the multiple sensors in order to halt a blasthole bit more precisely at a prescribed location, above a chosen horizon. Embodiments of the invention thus may provide the coal industry with an effective tool for reducing blasting coal damage losses by predicting the approaching coal seam during blast-hole drilling. Embodiments of the invention may be implemented on mine sites for routine use, which is expected to significantly improve blast hole drilling control and lead to reduced dilution, increased production, and profits in the open cut environment.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A method for sensing an interface between different layers during drilling, the method comprising: electromagnetically exciting a drill bit at the end of a drill string; detecting impedance changes near the drill bit during drilling; and processing detected impedance changes to determine a proximity of the drill bit to a targeted interface between different layers during drilling.

2. The method of claim 1 , wherein the layers are insulative and the electromagnetic excitations are in the HF - VHF range.

3. The method of claim 1 or claim 2, wherein the drill string is excited by a borehole radar (BHR) that inductively couples guided EM waves to the steel.

4. 4 The method of any one of claims 1 to 3, wherein the drill bit is excited by a near-monochromatic oscillator.

5. The method of any one of claims 1 to 4 wherein the EM excitations are coupled onto the drill string distal from the drill bit, for propagation to the drill bit.

6. The method of claim 5 wherein the EM excitations are coupled onto the drill string at or proximal to a borehole entry.

7. The method of claim 6 wherein the EM excitations are coupled onto the drill string by a coil mounted on a drill rig and positioned around a top of the drill string and configured to inductively couple EM signals to or from the drill string.

8. The method of any one of claims 1 to 4 wherein the EM excitations are coupled onto the drill string proximal to the drill bit.

9. The method of claim 5 wherein the EM excitations are coupled onto the drill string by driving a section of steel near the drill bit into resonance using a down-hole EM signal source.

10. The method of any one of claims 1 to 9 further comprising generating an alert signal to alert a driller operating the drill string of the proximity of the drill bit to an interface.

11. The method of any one of claims 1 to 10 wherein the formation is relatively conductive and the electromagnetic excitation is at a frequency of 200 kHz or less.

12. The method of any one of claims 1 to 11 , further comprising obtaining a down-hole measure of the current which actually passes through the drill bit.

13. The method of any one of claims 1 to 11 further comprising monitoring the output of accelerometers positioned near the bit to effect seismic geostopping.

14. The method of any one of claims 1 to 12, used for geostopping before the drill bit pierces an interface.

15. A system to sense an interface between different rock layers ahead of a drill bit during drilling, the system comprising: a transducer to electromagnetically excite a drill bit at the end of a drill string; a sensor to detect impedance changes near the drill bit during drilling; and a processor to process detected impedance changes to determine a proximity of the drill bit to a targeted interface between different layers during drilling.

16. The system of claim 14, wherein the layers are insulative and the transducer is configured to produce electromagnetic excitations in the HF - VHF range.

17. The system of claim 14 or claim 15, further comprising a borehole radar (BHR) that inductively couples guided EM waves to the steel to excite the drill string.

18. The system of any one of claims 14 to 16 wherein the EM excitations are coupled onto the drill string distal from the drill bit, for propagation to the drill bit.

19. The system of claim 17 wherein the EM excitations are coupled onto the drill string at or proximal to a borehole entry.

20. The system of claim 18 wherein the transducer is a coil mounted on a drill rig and positioned around a top of the drill string and configured to inductively couple EM signals to or from the drill string.

21. The system of any one of claims 14 to 16 wherein the EM excitations are coupled onto the drill string proximal to the drill bit.

22. The system of claim 20 wherein the EM excitations are coupled onto the drill string by driving a section of steel near the drill bit into resonance using a down-hole EM signal source.

23. The system of any one of claims 14 to 21 wherein the processor is further operable to generate an alert signal to alert a driller operating the drill string of the proximity of the drill bit to an interface.

24. The system of any one of claims 14 to 22 wherein the formation is relatively conductive and the electromagnetic excitation is at a frequency of 200 kHz or less.

25. The system of any one of claims 17 to 19, further comprising a current sensor for obtaining a down-hole measure of the current which actually passes through the drill bit.

26. The system of any one of claims 14 to 24 further comprising accelerometers positioned near the bit to effect seismic geostopping.